Many therapeutic agents are most effective when made available at constant rates at or near the absorption sites. The absorption of therapeutic agents thus made available generally results in desired plasma concentrations leading to maximum efficacy, and minimum toxic side effects. Much effort has been devoted to developing sophisticated drug delivery systems such as osmotic devices for oral application. However, there are instances where maintaining a constant blood level of a drug is not desirable. For example, a major objective of chronotherapy for cardiovascular diseases is to deliver the drug in higher concentrations during the time of greatest need, e.g., the early morning hours, and in lesser concentrations when the need is less, e.g., during the late evening and early sleep hours. In addition to a properly designed drug delivery system, the time of administration is equally important. The unique pharmacokinetic profile needed can be calculated from a simulated modeling developed using the pharmacokinetic parameters, knowledge of drug solubility, absorption along the gastrointestinal tract and elimination half-life.
A timed, pulsatile delivery system capable of providing one or more immediate release pulses at predetermined lag times or at specific sites result in better absorption of the active and more effective plasma profile. However, there are only a few such orally applicable pulsatile release systems due to potential limitations of the dosage form size, and/or polymeric materials and their compositions used for producing dosage forms. Ishino et al. disclose a dry-coated tablet form in Chemical Pharm. Bull. Vol. 40 (11), p 3036-3041 (1992). U.S. Pat. No. 4,871,549 assigned to Fujisawa Pharmaceutical Company discloses the preparation of a time-controlled explosion system in which rapid-release pulses at predetermined time intervals are caused by explosion of the membrane surrounding the drug cores comprising swelling agents such as disintegrants (e.g., low-substituted hydroxypropylcellulose, crospovidone, crosslinked carboxymethylcellulose, sodium starch glycolate). These systems are rather difficult to manufacture and do not consistently perform.
U.S. Pat. No. 6,531,152 discloses an explosion-controlled drug delivery system comprising a core containing a drug in combination with a core material (such as a polysaccharide or a crosslinked protein and a disintegrant that swell on exposure to body fluids or water) having a rigid membrane comprising hydrophobic and hydrophilic polymers that bursts rapidly releasing the active when the core swells. The '152 patent discloses specific tablet formulations having lag-times of up to about 12 hours. U.S. Pat. No. 6,287,599 to Burnside et al. discloses a pharmaceutical composition (a tablet formulation) comprising at least one pharmaceutically active agent that has a pH dependent solubility, at least one non-pH dependent sustained release agent and at least one pH-dependent agent that increases the dissolution rate of the active at a pH in excess of 5.5. Such a system exhibits approximately pH independent drug release profile.
However, monolithic drug delivery systems exhibit variable gastrointestinal transit times, and multiparticulate dosage forms containing coated drug particles (beads, pellets or micro-tablets) exhibiting consistent GI transit times are preferred.
The pulsatile burst release times in the above-described delivery systems are controlled by choosing appropriate core material, and by varying the membrane composition and/or thickness. However, it is difficult to consistently manufacture quality products based on such drug delivery systems wherein the drug-release is controlled by a swelling agent, a hydrophobic excipient, an osmotic agent alone or mixtures thereof.
U.S. Pat. No. 6,627,223, assigned to Eurand Pharmaceutical Limited, which is incorporated herein by reference, discloses a pulsatile release system consisting of a combination of one or more bead populations, each with a well-defined release profile. A timed, sustained-release profile (i.e., a sustained-release profile over a 12 to 24 hours after a lag-time of about 4 hours (i.e., a period of little or no release) following oral administration is disclosed in U.S. Pat. No. 6,500,454, and a biphasic release profile (i.e., an immediate-release pulse and a rapid burst after a lag-time of about 3 hours) is disclosed in U.S. Pat. No. 6,663,888. Although, a lag-time of greater than 3 hours could be achieved by applying a membrane comprising a water-insoluble polymer such as ethylcellulose (Ethocel Standard Premium 10 cps available from Dow Chemical Company) and an enteric polymer such as hydroxypropyl methylcellulose phthalate (HP-55 available from Shin-Etsu Chemical Corporation, Tokyo, Japan) on drug-layered beads containing propranolol hydrochloride (56% drug-load coated on 25-30 mesh sugar spheres) at 10-15% weight gain, the same coating composition applied on drug-layered beads containing nizatidine (56% drug-load coated on 25-30 mesh sugar spheres) even at 35-39% by weight resulted in a lag-time of less than 3 hours. It was considered in the prior art that the solubility of therapeutic agent in the dissolution medium and/or the molecular weight of the agent determined the drug dissolution within the coated bead and its diffusion out of the membrane. After extensive investigations, it was surprisingly discovered that apart from pH-dependent solubility of the therapeutic agent, its acidity/alkalinity has a significant effect on the lag-time that could be achieved. Additionally, the impact of a barrier coating (i.e., an intermediate coating applied in between the inner protective seal coat and the outer lag time coating, hereafter referred to as the barrier coat) and/or its composition on lag-time that could be achieved can vary depending on the acidity/alkalinity of the actives.